Position measurement by laser beam

Abstract

A laser beam from a source 20 is used to provide a reference in space adjacent a portion of a structure movements of which are monitored. The beam is received by a receiver 30. A number of targets 300 may be mounted to move with parts of the structure extending alongside the beam path. Target constructions (FIGS. 6-8) are described which allow for normal passage of the beam past the target but which enable the position of the beam relative to a given target to be measured by intercepting the beam. In a variation (FIG. 13) for civil engineering use, a single target is selectively mountable to one of a number of supports. Another variation (FIG. 11) uses graduated mesh targets on which the beam impinges on each mesh but is also transmitted through the mesh. Dynamic movement of a part of structure such as a bridge is measured with a single target attached to the part in question and comprising a barred reflective assembly (FIG. 3), a fibre optic array (FIG. 4) or a position sensing photo-diode (FIG. 5). For use in such systems there is also described a spatial filter for a laser receiver (FIG. 10), a laser assembly adapted for precise repositioning (FIG. 9) and a dust excluding laser housing (FIG. 12).

We claim: 
     
       1. Apparatus for montoring deflections of a structure relative to a laser beam, comprising: (a) a structure to be monitored;   (b) a laser mounted at a first location to direct a laser beam along a path extending adjacent a portion of the structure;   (c) at least one target mounted to said structure at a location adjacent the path of the laser beam, wherein said target comprises:   (d) a support portion fixed to the structure at the target location,   (e) at least one member having a beam intercepting edge movable with respect to said support portion between positions displaced from and intercepting the beam, respectively,   (f) actuator means acting between said support portion and said member to move said member between said positions,   (g) means for detecting the interception of the beam by said member, and   (h) means for measuring the position of the member relative to the support portion at beam interception.   
     
     
       2. Apparatus as claimed in claim 1, wherein said target comprises: (a) a further member having a beam intercepting edge movable with respect to said support portion between positions displaced from and intercepting the beam, respectively, said one member and said further member being movable in different directions;   (b) further actuator means acting between said support portion and said further member to move said further member between said positions, and   (c) further means for measuring the positions of said further member relative to the support portion at beam interception.   
     
     
       3. Apparatus as claimed in claim 1, wherein said beam intercepting edge and is movable along an axis normal to said edge is a straight edge and to the laser beam. 
     
     
       4. Apparatus as claimed in claim 3, wherein said edge comprises a plurality of means spaced therealong to detect the interception of the laser beam thereby, anad provide a measure of the beam position relative to the target in the direction of said edge. 
     
     
       5. Apparatus as claimed in claim 1, wherein said interception detection means comprises a laser receiver at a location remote from said laser and beyond said target. 
     
     
       6. Apparatus as claimed in claim 2, wherein the beam intercepting edges of said one member and said further member are both straight edges, and said straight edges lie at right angles to one another, each of said one member and said further member being individually movable in a direction normal to its beam intercepting edge and to the laser beam. 
     
     
       7. Apparatus as claimed in claim 6, wherein said interception detection means comprises a laser receiver at a location remote from said laser and beyond said target. 
     
     
       8. Apparatus as claimed in claim 32, wherein each of said straight edges is provided with respective means for detecting the interception of a laser beam thereby. 
     
     
       9. Apparatus as claimed in claim 1, wherein the actuator means is manually driven. 
     
     
       10. Apparatus as claimed in claim 1, wherein the actuator means comprises a motor drive. 
     
     
       11. Apparatus as claimed in claim 10, comprising a plurality of targets secured to said structure at spaced locations adjacent the beam path, and control means for successively actuating the actuating means for each target such that successive targets provide measurements and are maintained in a non-beam intercepting position when not being actuated for measurement. 
     
     
       12. Apparatus as claimed in claim 1, wherein said member comprises two parallel, spaced apart beam intercepting edges movable together such that the beam passes between the edges, and beam interception detecting means on each edge for detecting relative deflectios of the beam toward one or another edge. 
     
     
       13. Apparatus as claimed in claim 12, wherein the beam interception detecting means on one edge comprises means for detecting the position of the beam along said one edge.